Field of Technology
The present application relates to a lighting device with a lighting unit including several light sources having different color spectra, with a sensor for determining the spectral power distribution (SPD) emitted by the lighting unit, with a control unit which, as a function of a predetermined spectral power distribution as well as of the spectral power distribution measured by the sensor, acts upon a drive unit which individually energizes the light sources of the lighting unit, so that the emitted light has the predetermined spectral power distribution. In this context, color spectrum denotes the electromagnetic waves of a range of defined bandwidth and intensity in the color space visually perceivable to humans.
The present application further relates to a method for operating a lighting device with a lighting unit which includes at least four light sources having different color spectra, with a sensor for determining the spectral power distribution (SPD) emitted by the lighting unit, a control unit which, as a function of a predetermined spectral power distribution as well as of the spectral power distribution measured by the sensor, acts upon a drive unit which individually energizes the light sources of the lighting unit, so that the emitted light has the predetermined spectral power distribution.
Brief Description of Related Art
In semiconductor based lighting elements, such as LEDs, the color spectrum and the brightness (intensity) change with increasing operation duration, which can be perceived as interference unless compensation is provided for this interference. In addition, LEDs are also affected by a dispersion of their technical properties with regard to brightness and color during manufacture. This is compensated for by the manufacturer using so-called “binning,” in which semiconductor elements are sorted according to a predetermined dispersion. The narrower the dispersion selection, the more expensive are the LEDs.
A device is known from EP 1 461 982 B1, in which a desired light color is generated from three LED light sources with red, green, and blue color spectra. In the process, the light emitted by the three LEDs is detected by a three-section filter, the measured RGB value is converted to the so-called CIE XYZ color space (CIE=Commission internationale de l'éclairage [International Commission on Illumination]). This measured value vector is compared in a control unit which functions as a P controller with an XYZ target value, which, depending on the error, acts upon a drive unit, which controls the electrical power supplied to the light sources accordingly. By means of such a device, compensation for such changes in the brightness and color can be provided.
However, the disadvantage here is that, on the one hand, the sensor has to be adjusted to the frequency spectra of the LEDs for the control unit to function sufficiently. Furthermore, with this system, a lighting device with more than three light sources having different color spectra—for example, a yellow or white LED as fourth LED—can no longer be controlled, because the result of this control is no longer unequivocal, since several luminosity settings of the four light sources can generate the same color impression in the XYZ color space.
DE 10 2007 044 556 describes a method for determining the light current components of individual LEDs via a v(lambda)-adapted sensor. The operationally conditioned color and brightness changes of the individual LEDs are determined by a measuring of the spectral component with the aid of a v(lambda)-adapted sensor and the measuring of the operating temperature of the LED (board and junction temperature). These measured values are determined individually for the particular controlled LED. The measured values then flow as input parameters of the determination of the individual emission spectra to the LED that can then be optimized regarding light current so that the entire light achieves a defined color and brightness. This has the disadvantage that only one individual light source can always be observed by the measuring method used. Even a detection of the color shift of an individual light source can be determined only indirectly with the information of the temperature and of the v(lambda) measuring. Non-temperature-dependent color changes of the light source cannot be differentiated with this from a change in brightness. It is also disadvantageous that the described adjustment of the color and brightness values of the light function only in one operating state in which the individual light sources are adjusted individually. This equals an interruption of the operation.